1. Field of the Invention
The present invention relates to piezoelectric resonators, and in particular, the present invention relates to a piezoelectric resonator for use principally in oscillators.
2. Description of the Related Art
Conventionally, an energy trap type piezoelectric resonator has been included in oscillators which utilize a longitudinal thickness mode fundamental wave of a piezoelectric single-chip, the third longitudinal thickness mode harmonic of a piezoelectric single-chip, a shear mode vibration of a piezoelectric single-chip, or a longitudinal thickness mode harmonic of a monolithic piezoelectric body.
However, in a conventional resonator utilizing a longitudinal thickness mode fundamental wave, materials capable of trapping energy are limited. Thus, it is difficult to manufacture an energy trap type resonator using a material having high thermal resistance. Although a conventional shear mode resonator can use a material having high thermal resistance, handling of high frequency resonators during production is complicated due to a high speed of sound, and the mechanical reliability of the element itself is insufficient. Further, these resonators have a large electromechanical coefficient and cannot be used in applications requiring narrow tolerances.
Elements utilizing the third longitudinal thickness mode harmonic of a piezoelectric single-chip do not have these problems. However, the optimum electrode diameter of the energy trap type resonator is relatively large. Therefore, the size of these elements cannot be sufficiently reduced. Also, the element thickness is three times that of the fundamental wave type. Thus, the thickness of the element also cannot be sufficiently reduced. The resonators utilizing a longitudinal thickness mode harmonic of a monolithic piezoelectric body can be constituted by materials having high thermal resistance while the optimum electrode diameter of the energy trap type resonator is similar to that of the longitudinal thickness mode fundamental wave resonator. Therefore the size can be reduced. However, these resonators also have a large electromechanical coefficient, and thus cannot be used in applications requiring narrow tolerances. As described above, by conventional techniques, compact oscillators having high thermal resistance and a narrow tolerance have been very difficult to achieve.
Japanese Unexamined Patent Application Publication No. 5-48377 discloses a piezoelectric resonator using a deposited material composed of two layers of a paraelectric layer without piezoelectricity and a piezoelectric layer. In this piezoelectric resonator, because an electric field is applied to the piezoelectric layer and the paraelectric layer which are connected in series, the piezoelectric layer is not effectively vibrated, and the characteristics of the resonator are substantially altered because a capacitive component is inserted in series in an equivalent circuit.
To overcome the above-described problems, preferred embodiments of the present invention provide a piezoelectric resonator which achieves a compact oscillator with high performance having high thermal resistance and a narrow tolerance.
According to a preferred embodiment of the present invention, a piezoelectric resonator includes at least one pair of vibration electrodes, and an element body including at least one excitation layer sandwiched between the at least one pair of vibration electrodes and excited by an electric field so as to be vibrated, and at least one non-excitation layer not excited so as to be vibrated, the piezoelectric resonator exciting a longitudinal thickness mode harmonic of the n-th order (n is an integer other than 1), wherein when the thickness of the element body is denoted by t and a unit layer thickness is represented by approximately t/n, the thickness of the excitation layer is an integer multiple of the unit layer thickness and the thickness of the at least one non-excitation layer is an integer multiple of the unit layer thickness.
In such a piezoelectric resonator, it is preferable that the unit layer thickness of the excitation layer is in the range of about 0.7 t/n to about 1.2 t/n and the unit layer thickness of the non-excitation layer is in the range of about 0.8 t/n to about 1.3 t/n.
Also, the vibration electrodes are provided only on one principal plane of the non-excitation layer, or vibration electrodes having the same potential may be provided on both principal planes of the non-excitation layer.
Furthermore, the non-excitation layer is made of non-polarized piezoelectric ceramics or dielectric ceramics.
Such a piezoelectric resonator further includes a first and a second end surface electrode provided on the surface of the element body, a spurious response suppressing electrode electrically connected to the first end surface electrode, wherein the spurious response suppressing electrode is configured to have a constant gap at one end of the vibrating electrode connected to the second end surface at the other end in a direction that is substantially perpendicular to the depositing direction of the excitation layer and the non-excitation layer.
Preferably, a ratio between the gap and the unit layer thickness, i.e., gap/unit layer thickness, is in the range of about 1.0 to about 3.0.
By applying an electric field between vibration electrodes defined by sandwiching the excitation layer, the excitation layer is excited to be vibrated in a longitudinal thickness mode. At this time, since the electric field is not applied to the non-excitation layer or the non-excitation layer is made of a material which cannot be excited even when an electric field is applied thereto, the non-excitation layer is not excited to be vibrated. However, a standing wave is also transmitted to the non-excitation layer by the vibration of the excitation layer, so that the entirety becomes a piezoelectric resonator utilizing a longitudinal thickness mode harmonic of a higher order. That is, when the thickness of the element body is denoted by t and a unit layer thickness is represented by approximately t/n, by multiplying the thickness of an excitation layer and at least one of non-excitation layers by an integer multiple of the unit layer thickness t/n, a piezoelectric resonator utilizing a longitudinal thickness mode harmonic of the n-th order as the entirety is obtained.
Such a piezoelectric resonator uses a material having high thermal resistance and reduces the value of an electromechanical coefficient, such that a resonator having a narrow tolerance is achieved.
In such a piezoelectric resonator, when the unit layer thickness of the excitation layer is in the range of about 0.7 t/n to about 1.2 t/n and the unit layer thickness of the non-excitation layer is in the range of about 0.8 t/n to about 1.3 t/n, a piezoelectric resonator with excellent characteristics is obtained.
A non-excitation layer is configured so that an electric field cannot be applied thereto by constructing it such that it is not sandwiched by vibration electrodes.
Also, a non-excitation layer is not excited even when an electric field is applied thereto by using non-polarized piezoelectric ceramics or dielectric ceramics. In this case, of course, the electrodes may be arranged so that an electric field cannot be applied to the non-excitation layer.
Furthermore, by forming a spurious response suppressing electrode, vibrations in any mode other than a desired mode are greatly suppressed.
These effects are remarkable when a ratio gap/unit layer thickness is in the range of about 1.0 to about 3.0.
These and other objects, features, and advantages of the present invention will become more apparent from the following detailed description of embodiments according to the present invention taken in connection with the drawings.